Fracturing is a process that results in the creation of fractures in rocks. The technique of fracturing (or “fracking”) is used to increase or restore the rate at which fluids, such as oil, gas or water, can be produced from a reservoir, including unconventional reservoirs such as shale rock or coal beds. Fracturing may facilitate the production of natural gas and oil from rock formations deep below the Earth's surface (e.g., 5,000-20,000 feet or 1,500-6,100 m). At such depth, there may not be sufficient porosity and permeability to allow natural gas and oil to flow from the rock into the wellbore at economic rates. The fractures produced by fracking, however, provide a conductive path connecting a larger area of the reservoir to the well, thereby increasing the area from which natural gas or liquid can be recovered from the targeted formation.
Hydraulic fracturing may be conducted by pumping the fracturing fluid into the wellbore at a rate sufficient to increase the pressure within the well to a value in excess of the fracture gradient of the formation rock. The pressure causes the formation to crack, allowing the fracturing fluid to enter and extend the crack farther into the formation. Hydraulic fracture stimulation is commonly applied to wells drilled in low-permeability reservoirs.
The location of fracturing along the length of the wellbore may be controlled by stimulation valves positioned below and/or above the region to be fractured. This allows a wellbore to be progressively fractured along the length of the wellbore, sometimes referred to as “multi-stage fracking” Piping above the valves admits fracturing fluid and proppant into the working region, while the valves may prevent such fluid (and pressure) from communicating below the region to be fractured. These stimulation valves typically use ball seats and plug elements.
Generally, such ball seat valves have progressively smaller ball seats as proceeding farther into the wellbore from the surface. This allows selective actuation (sealing) of the stimulation valve by deploying progressively larger balls. The initially very small balls pass by the valves at the top, and are caught by the largest valve with a seat small enough to catch the ball. While this has been successfully implemented many times, the design calls for stimulation valves with many different sizes, which complicates the fracturing assembly. Other challenges also arise in such assemblies.